3.4 BuildRequest

A BuildRequest is a request to build a specific revision of the source
code (specified by a source stamp) on a single Builder. Each
Builder runs the BuildRequest as soon as it can (i.e. when an
associated buildslave becomes free). BuildRequests are prioritized from
oldest to newest, and when a buildslave becomes free, the Builder with
the oldest BuildRequest is run.

The BuildRequest may be mergeable with other compatible
BuildRequests. Builds that are triggered by incoming Changes
will generally be mergeable. Builds that are triggered by user
requests are generally not, unless they are multiple requests to build
the latest sources of the same branch.